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Scintillation and optical stimulated luminescence of Ce-doped CaF2
Radiation Measurements xxx (2014) 1e4
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Scintillation and optical stimulated luminescence of Ce-doped CaF2 Takayuki Yanagida a, *, Yutaka Fujimoto a, Kenichi Watanabe b, Kentaro Fukuda c, Noriaki Kawaguchi d, Yuka Miyamoto e, Hidehito Nanto f a
Kyushu Institute of Technology, Frontier Research Academy for Young Researchers, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196 Japan Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan c Tokuyama Corporation, Shibuya 3-chome, Shibuya-ku, Tokyo 150-8383 Japan d A&T Corporation, 2023e1 Endo, Fujisawa, Kanagawa 252e0816 Japan e Chiyoda Technol Corporation, 3681 Narita-cho, Oarai-machi, Higashi-ibaraki-gun, Ibaraki 311-1313, Japan f Kanazawa Institute of Technology, 3-1 Yatsukaho, Hakusann-shi, Ishikawa 924-0838, Japan b
h i g h l i g h t s Optical, scintillation, and OSL properties of Ce 0.1e20% doped CaF2 were studied. Scintillation light yield exhibited inverse proportionality to Ce concentrations. OSL intensities showed proportionality to Ce concentrations. Complementary relation of scintillation and OSL was experimentally confirmed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 November 2013 Received in revised form 20 January 2014 Accepted 25 March 2014
Scintillation and optical stimulated luminescence of Ce 0.1e20% doped CaF2 crystals prepared by Tokuyama Corp. were investigated. In X-ray induced scintillation spectra, luminescence due to Ce3þ 5d e4f transition appeared around 320 nm with typically 40 ns decay time. By 241Am 5.5 MeV a-ray irradiation, 0.1% doped one showed the highest scintillation light yield and the light yield monotonically decreased with Ce concentrations. Optically stimulated luminescence after X-ray irradiation was observed around 320 nm under 550 or 830 nm stimulation in all samples. As a result, intensities of optically stimulated luminescence were proportional to Ce concentrations. Consequently, scintillation and optically stimulated luminescence resulted to have a complementary relation in Ce-doped CaF2 system. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Scintillation Scintillator OSL Ce3þ CaF2
1. Introduction In ionizing radiation detectors, luminescent materials were used to visualize invisible radiations by visible photons. Among such luminescent materials, scintillators which convert a single high energy ionizing radiation to thousands of ultravioletevisible photons immediately (Yanagida, 2013) is the most famous type and has a wide spread applications such as medical (Yanagida et al., 2010a,b,c), security (Totsuka et al., 2011), well-logging (Yanagida et al., 2013aed), and high energy physics (Yamaoka et al., 2005; Ito et al., 2007). On the other hand, another luminescent type detector, dosimeter storages the absorbed energy for several weeks
* Corresponding author. Tel./fax: þ81 93 695 6049. E-mail address: [email protected] (T. Yanagida).
and release the energy by optical (optically stimulated luminescence, OSL) or thermal (thermally stimulated luminescence, TSL) stimulation. The main application of OSL (Yukihara and McKeever, 2011) or TSL (McKeever, 1985) dosimeters is a personal dosimetry to measure ionizing radiation. Historically, scintillators and dosimeters were investigated and developed by different communities due to differences of detection techniques and time scales while their purpose was the same, to detect ionizing radiation by luminescent materials. However, if we assume the energy conservation, scintillation and OSL/TSL should be complementary in some material systems. When some energies are absorbed in materials, some parts are released by luminescence immediately (scintillation). The simple question occurs why some scintillators are bright and the others are not. In other words, where remaining energies has gone in dark scintillator materials? In this case, some portion of remaining energies is storage in the material
http://dx.doi.org/10.1016/j.radmeas.2014.03.020 1350-4487/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yanagida, T., et al., Scintillation and optical stimulated luminescence of Ce-doped CaF2, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.03.020
2
T. Yanagida et al. / Radiation Measurements xxx (2014) 1e4
and they can be released by stimulation (OSL or TSL). In other cases, remaining portions of energy are converted to thermal energies which are used in calorimeter for radiation detection. Therefore we suppose scintillation and OSL/TSL can be treated unity concerning the energy income and expenditure in some material systems, and the understanding of this relation will be a great benefit for future scintillator/dosimeter developments. In the present work, we examine the hypothesis that scintillation and OSL have a complementary relation in some material systems experimentally. For this purpose, Ce 0.1, 0.5, 1, 3, 5, 10, and 20% doped Ce3þ-doped CaF2 crystals were prepared by Tokuyama Corp. using the simple solidification method (Kawaguchi et al., 2013). The reason to choose CaF2:Ce is that it can emit detectable scintillation and OSL signals. Fig. 1 demonstrates samples for evaluations. They all had similar sizes, typically 10 mm diameter and 1 mm thickness. 2. Experimental procedures To evaluate basic optical property, optical in-line transmittance was evaluated by using JASCO V670 spectrometer. Then, as basic scintillation properties, X-ray induced radioluminescence spectra (Yanagida et al., 2013a) and decay time profiles (Yanagida et al., 2010b, 2013b) were investigated. The aim of this work was to investigate a basic relation of scintillation and OSL. For this purpose, the confirmation that our samples had typical scintillation responses of CaF2:Ce was important. Scintillation light yield were determined by pulse height with our standard setup (Yanagida et al., 2013c). Because g-ray sensitivity of CaF2 was not so high due to low effective atomic number (Zeff ¼ 16.4) and density (3.2 g/ cm3), 241Am 5.5 MeV a-ray (4 MBq) was selected as the excitation source. OSL properties were evaluated by photoluminescence (PL) quantum yield (QY) with Hamamatsu Quantaurus-QY after X-ray exposure (0.5e4 Gy). The bias voltage and tube current of X-ray generator were 40 kV and 5.2 mA, respectively. The exposure dose was evaluated by X-ray ionization chamber. The absolute QY was calculated by the following equation, QY ¼ Nemit/Nabsorb where Nemit and Nabsorb were number of emission and absorption photons, respectively (e.g., Yanagida et al., 2013d). One of the problems of OSL evaluations is that OSL intensity is a qualitative value while irradiated dose is quantitative. In this paper, we used PL QY as the OSL intensity because it was largely used to evaluate emission intensity of general phosphors as well as luminosity (Cd/cm2).
Fig. 2. Transmittance spectra of Ce-doped CaF2 crystals in the wavelength range from 190 to 320 nm.
doped CaF2 scintillators. Luminescence (300e360 nm) peaks were originated from Ce3þ 5de4f transition. In low Ce concentration samples, self-trapped exciton luminescence at 270 nm was observed. A sharp line around 300 nm in Ce 20% doped sample was a noise due to a direct hit of X-ray or background cosmic ray to CCD. Fig. 4 demonstrates the relation between scintillation decay times and Ce concentrations. Scintillation decay times monotonically became faster when the Ce concentrations increased. Finally, scintillation decay time reached w30 ns in Ce 20% doped sample. Based on these optical and scintillation evaluations, it was confirmed that our Cedoped CaF2 crystals exhibited typical properties of this material system. To evaluate scintillation light yield, all samples were irradiated with 5.5 MeV a-rays of 241Am. Fig. 5 exemplifies relation between the scintillation light yield and the dopant concentration. As a representative, the pulse height spectrum of Ce 0.1% doped sample is demonstrated in the inset and a-ray full energy peak was clearly observed. Taking into account the quantum efficiency of the used PMT around 320 nm, scintillation light yield was deduced. Scintillation light yields monotonically decreased with Ce concentrations and this tendency was consistent with previous result (Gektin et al., 2009; Yanagida et al., 2010c).
3. Results and discussion
Intensity (a. u.)
Fig. 2 represents optical in-line transmittance spectra of Cedoped CaF2 from 190 to 320 nm. Absorption bands at 200, 240, and 300 nm were ascribed to Ce3þ 4fe5d transition and absorption depths were in proportion to nominal Ce concentrations. Transmittance at wavelength longer than 350 nm was typically 80e90% without noticeable absorption lines. Then, we investigated basic luminescence properties. Fig. 3 represents X-ray induced radioluminescence spectra of Ce differently
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Fig. 1. Ce-doped CaF2 crystals illuminated by UV-lamp.
Fig. 3. X-ray induced radioluminescence spectra of Ce-doped CaF2 scintillators in the wavelength range from 180 to 700 nm. The supplied bias voltage and tube current were 80 kV and 1 mA, respectively. The X-ray exposure time was 10 s for each integration.
Please cite this article in press as: Yanagida, T., et al., Scintillation and optical stimulated luminescence of Ce-doped CaF2, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.03.020
T. Yanagida et al. / Radiation Measurements xxx (2014) 1e4
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doped CaF2 exhibited the highest OSL intensity since it had a large number of defects due to a charge imbalance of Ce3þ and Ca2þ. Then in Fig. 7, the relation between the OSL intensity after 2 Gy X-ray exposure and the dopant concentration is shown. Unlike the similar plot in scintillation light yield, OSL intensity monotonically increased with the Ce concentration. Finally, the relation between the scintillation light yield under 241 Am a-ray irradiation and OSL intensities after 0.5e4 Gy X-ray exposure is demonstrated in Fig. 8. Though it was obvious from Figs. 5 and 7, a reverse proportional relation between the scintillation light yield and the OSL intensity was clearly observed and our assumption of the complementary relation of scintillation and OSL was experimentally confirmed in Ce-doped CaF2 system. If we used X- or g-ray as an excitation source to evaluate scintillation light yield, the observed relation will be similar since charged particle and high energy photon induced light yields are in proportion but the energy conversion efficiency is different. This conversion efficiency is called a/b (or a/g) ratio and this ratio shows little dependence on dopant concentrations (e.g., the case in LuAG:Pr, Yanagida et al., 2012). Except Ce 20% doped one, OSL intensities
Stimulation wavelength (nm)
Fig. 6 depicts OSL emission map of Ce 20% doped CaF2. OSL appeared around 320 nm with 550 and 830 nm stimulation. Throughout OSL evaluations, QY was deduced by summing up these 550 and 830 nm emission peaks. Among present samples, Ce 20% 500
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Fig. 6. OSL emission map of Ce 20% doped CaF2. The horizontal axis shows the stimulation wavelength and the vertical axis exhibits emission wavelength with 10 nm step.
Fig. 8. Relation between the scintillation light yield (ph/5.5 MeV-a) and OSL intensity (QY, %) after 0.5, 1, 2, and 4 Gy X-ray exposure.
Please cite this article in press as: Yanagida, T., et al., Scintillation and optical stimulated luminescence of Ce-doped CaF2, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.03.020
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T. Yanagida et al. / Radiation Measurements xxx (2014) 1e4
systematically decreased when X-ray exposure increased and it was suggested that OSL responses in Ce 0.1e1% doped samples saturated around few Gy exposure. Simply, OSL response was improved in higher Ce concentration sample by creation of defects due to the charge imbalance. Of course, the complementary relation of scintillation and OSL is not always observed in all material systems because there are some competing processes with OSL/TL in energy storage processes of dosimeter materials (e.g., tunnelling). In such material systems, most part of the absorbed energy would be converted to thermal form and we could not observe the complementary relation in such material systems. 4. Conclusion Ce 0.1e20% doped CaF2 crystals were prepared by Tokuyama Corp. using the simple solidification method. By optical transmittance, radioluminescence, and scintillation decay, it was confirmed that Ce-doped CaF2 used in this work exhibited typical properties of this material system. Scintillation light yield had an inverse proportionality to Ce concentrations while OSL intensity was in proportion to dopant concentrations. The hypothesis that the scintillation and OSL had a complementary relation in some material systems was confirmed experimentally. Acknowledgements This work was mainly supported by JST Sentan, A-step and partially by a Grant in Aid for Young Scientists (A) e 23686135, and Challenging Exploratory Research e 23656584 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese government. Partial assistance from Nippon Sheet Glass Foundation for Materials Science and Engineering, Tokuyama Science Foundation, Iketani Science and Technology Foundation, Hitachi Metals Materials Science Foundation, Mazda Foundation, JFE 21st Century Foundation, and The Asahi Glass Foundation, are also gratefully acknowledged. References Gektin, A.V., Shiran, N., Nesterkina, V., Boyarintseva, Y., Baumer, V., Stryganyuk, G., Shimamura, K., Villora, E., 2009. Luminescence of heavily ce-doped alkalineearth fluorides. IEEE Trans. Nucl. Sci. 56, 1002e1005.
Ito, T., Yanagida, T., Sato, M., Kokubun, M., Takashima, T., Hirakuri, S., Miyawaki, R., Takahashi, H., Makishima, K., Tanaka, T., Nakazawa, K., Takahashi, T., Honda, T., 2007. A 1-dimensional gamma-ray position sensor based on GSO:Ce scintillators coupled to a Si strip detector. Nucl. Instr. Methods 579, 239e242. Kawaguchi, N., Yanagida, T., Futami, Y., Fujimoto, Y., Fukuda, K., Kajimoto, S., Fukumura, H., 2013. Doping concentration dependence on VUV luminescence of Tm:CaF2. Opt. Mater. 35, 1898e1901. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press. Totsuka, D., Yanagida, T., Fukuda, K., Kawaguchi, N., Fujimoto, Y., Yokota, Y., Yoshikawa, A., 2011. Performance test of Si PIN photodiode line scanner for thermal neutron detection. Nucl. Instrum. Methods A 659, 399e402. Yamaoka, K., Ohno, M., Terada, Y., Hong, S., Kotoku, J., Okada, Y., Tsutsui, A., Endo, Y., Abe, K., Fukazawa, Y., Hirakuri, S., Hiruta, T., Itoh, K., Kamae, T., Kawaharada, M., Kawano, N., Kawashima, K., Kishishita, T., Kitaguchi, T., Kokubun, M., Madejski, G.M., Makishima, K., Mitani, T., Miyawaki, R., Murakami, T., Murashima, M.M., Nakazawa, K., Niko, H., Nomachi, M., Oonuki, K., Sato, G., Suzuki, M., Takahashi, H., Takahashi, I., Takahashi, T., Takeda, S., Tamura, T., Tanaka, T., Tashiro, M., Watanabe, S., Yanagida, T., Yonetoku, 2005. Development of the HXD-II wideband all-sky monitor onboard astro-E2. IEEE. Trans. Nucl. Sci. 52, 2765e2772. Yanagida, T., Yoshikawa, A., Yokota, Y., Kamada, K., Usuki, Y., Yamamoto, S., Miyake, M., Baba, M., Sasaki, K., Ito, M., 2010a. Development of Pr:LuAG scintillator array and assembly for positron emission mammography. IEEE. Nucl. Trans. Sci. 57, 1492e1495. Yanagida, T., Fujimoto, Y., Yoshikawa, A., Yokota, Y., Kamada, K., Pejchal, J., Kawaguchi, N., Fukuda, K., Uchiyama, K., Mori, K., Kitano, K., Nikl, M., 2010b. Development and performance test of picosecond pulse X-ray excited streak camera system for scintillator characterization. Appl. Phys. Express 3, 056202. Yanagida, T., Kim, K.J., Kamada, K., Yokota, Y., Maeo, S., Yoshikawa, A., Kawaguchi, N., Fukuda, K., Sarukura, N., Chani, V., 2010c. Growth, optical properties, and scintillation light yield of Ce:CaF2 crystals with different Ce concentration. Jpn. J. Appl. Phys. 49, 032601. Yanagida, T., Fujimoto, Y., Kamada, K., Totsuka, D., Yagi, H., Yanagitani, T., Futami, Y., Yanagida, S., Kurosawa, S., Yokota, Y., Yoshikawa, A., Nikl, M., 2012. Scintillation properties of transparent ceramic Pr:LuAG for different Pr concentration. IEEE Trans. Nucl. Sci. 59, 2146. Yanagida, T., 2013. Study of rare-earth-doped scintillators. Opt. Mater. 35, 1987e 1992. Yanagida, T., Kamada, K., Fujimoto, Y., Yagi, H., Yanagitani, T., 2013a. Comparative study of ceramic and single crystal Ce:GAGG scintillator. Opt. Mater. 35, 2480e 2485. Yanagida, T., Fujimoto, Y., Yamaji, A., Kawaguchi, N., Kamada, K., Totsuka, D., Fukuda, K., Yamanoi, K., Nishi, R., Kurosawa, S., Shimizu, T., Sarukura, N., 2013b. Study of the correlation of scintillation decay and emission wavelength. Radiat. Meas. 55, 99e102. Yanagida, T., Kawaguchi, N., Fujimoto, Y., Fukuda, K., Watanabe, K., Yamazaki, A., Uritani, A., 2013c. Scintillation properties of LiFeSrF2 and LiFeCaF2 eutectic. J. Lumin. 144, 212e216. Yanagida, T., Fujimoto, Y., Fukuda, K., Chani, V., 2013d. Optical and scintillation properties of Ce-doped LuLiF4 with different Ce concentrations. Nucl. Instrum. Methods Phys. Res. A 729, 58e63. Yukihara, E.G., McKeever, S.W.S., 2011. Optically Stimulated Luminescence: Fundamentals and Applications. Wiley.
Please cite this article in press as: Yanagida, T., et al., Scintillation and optical stimulated luminescence of Ce-doped CaF2, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.03.020
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Scintillation and optical stimulated luminescence of Ce-doped CaF2 Takayuki Yanagida a, *, Yutaka Fujimoto a, Kenichi Watanabe b, Kentaro Fukuda c, Noriaki Kawaguchi d, Yuka Miyamoto e, Hidehito Nanto f a
Kyushu Institute of Technology, Frontier Research Academy for Young Researchers, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196 Japan Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan c Tokuyama Corporation, Shibuya 3-chome, Shibuya-ku, Tokyo 150-8383 Japan d A&T Corporation, 2023e1 Endo, Fujisawa, Kanagawa 252e0816 Japan e Chiyoda Technol Corporation, 3681 Narita-cho, Oarai-machi, Higashi-ibaraki-gun, Ibaraki 311-1313, Japan f Kanazawa Institute of Technology, 3-1 Yatsukaho, Hakusann-shi, Ishikawa 924-0838, Japan b
h i g h l i g h t s Optical, scintillation, and OSL properties of Ce 0.1e20% doped CaF2 were studied. Scintillation light yield exhibited inverse proportionality to Ce concentrations. OSL intensities showed proportionality to Ce concentrations. Complementary relation of scintillation and OSL was experimentally confirmed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 November 2013 Received in revised form 20 January 2014 Accepted 25 March 2014
Scintillation and optical stimulated luminescence of Ce 0.1e20% doped CaF2 crystals prepared by Tokuyama Corp. were investigated. In X-ray induced scintillation spectra, luminescence due to Ce3þ 5d e4f transition appeared around 320 nm with typically 40 ns decay time. By 241Am 5.5 MeV a-ray irradiation, 0.1% doped one showed the highest scintillation light yield and the light yield monotonically decreased with Ce concentrations. Optically stimulated luminescence after X-ray irradiation was observed around 320 nm under 550 or 830 nm stimulation in all samples. As a result, intensities of optically stimulated luminescence were proportional to Ce concentrations. Consequently, scintillation and optically stimulated luminescence resulted to have a complementary relation in Ce-doped CaF2 system. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Scintillation Scintillator OSL Ce3þ CaF2
1. Introduction In ionizing radiation detectors, luminescent materials were used to visualize invisible radiations by visible photons. Among such luminescent materials, scintillators which convert a single high energy ionizing radiation to thousands of ultravioletevisible photons immediately (Yanagida, 2013) is the most famous type and has a wide spread applications such as medical (Yanagida et al., 2010a,b,c), security (Totsuka et al., 2011), well-logging (Yanagida et al., 2013aed), and high energy physics (Yamaoka et al., 2005; Ito et al., 2007). On the other hand, another luminescent type detector, dosimeter storages the absorbed energy for several weeks
* Corresponding author. Tel./fax: þ81 93 695 6049. E-mail address: [email protected] (T. Yanagida).
and release the energy by optical (optically stimulated luminescence, OSL) or thermal (thermally stimulated luminescence, TSL) stimulation. The main application of OSL (Yukihara and McKeever, 2011) or TSL (McKeever, 1985) dosimeters is a personal dosimetry to measure ionizing radiation. Historically, scintillators and dosimeters were investigated and developed by different communities due to differences of detection techniques and time scales while their purpose was the same, to detect ionizing radiation by luminescent materials. However, if we assume the energy conservation, scintillation and OSL/TSL should be complementary in some material systems. When some energies are absorbed in materials, some parts are released by luminescence immediately (scintillation). The simple question occurs why some scintillators are bright and the others are not. In other words, where remaining energies has gone in dark scintillator materials? In this case, some portion of remaining energies is storage in the material
http://dx.doi.org/10.1016/j.radmeas.2014.03.020 1350-4487/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yanagida, T., et al., Scintillation and optical stimulated luminescence of Ce-doped CaF2, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.03.020
2
T. Yanagida et al. / Radiation Measurements xxx (2014) 1e4
and they can be released by stimulation (OSL or TSL). In other cases, remaining portions of energy are converted to thermal energies which are used in calorimeter for radiation detection. Therefore we suppose scintillation and OSL/TSL can be treated unity concerning the energy income and expenditure in some material systems, and the understanding of this relation will be a great benefit for future scintillator/dosimeter developments. In the present work, we examine the hypothesis that scintillation and OSL have a complementary relation in some material systems experimentally. For this purpose, Ce 0.1, 0.5, 1, 3, 5, 10, and 20% doped Ce3þ-doped CaF2 crystals were prepared by Tokuyama Corp. using the simple solidification method (Kawaguchi et al., 2013). The reason to choose CaF2:Ce is that it can emit detectable scintillation and OSL signals. Fig. 1 demonstrates samples for evaluations. They all had similar sizes, typically 10 mm diameter and 1 mm thickness. 2. Experimental procedures To evaluate basic optical property, optical in-line transmittance was evaluated by using JASCO V670 spectrometer. Then, as basic scintillation properties, X-ray induced radioluminescence spectra (Yanagida et al., 2013a) and decay time profiles (Yanagida et al., 2010b, 2013b) were investigated. The aim of this work was to investigate a basic relation of scintillation and OSL. For this purpose, the confirmation that our samples had typical scintillation responses of CaF2:Ce was important. Scintillation light yield were determined by pulse height with our standard setup (Yanagida et al., 2013c). Because g-ray sensitivity of CaF2 was not so high due to low effective atomic number (Zeff ¼ 16.4) and density (3.2 g/ cm3), 241Am 5.5 MeV a-ray (4 MBq) was selected as the excitation source. OSL properties were evaluated by photoluminescence (PL) quantum yield (QY) with Hamamatsu Quantaurus-QY after X-ray exposure (0.5e4 Gy). The bias voltage and tube current of X-ray generator were 40 kV and 5.2 mA, respectively. The exposure dose was evaluated by X-ray ionization chamber. The absolute QY was calculated by the following equation, QY ¼ Nemit/Nabsorb where Nemit and Nabsorb were number of emission and absorption photons, respectively (e.g., Yanagida et al., 2013d). One of the problems of OSL evaluations is that OSL intensity is a qualitative value while irradiated dose is quantitative. In this paper, we used PL QY as the OSL intensity because it was largely used to evaluate emission intensity of general phosphors as well as luminosity (Cd/cm2).
Fig. 2. Transmittance spectra of Ce-doped CaF2 crystals in the wavelength range from 190 to 320 nm.
doped CaF2 scintillators. Luminescence (300e360 nm) peaks were originated from Ce3þ 5de4f transition. In low Ce concentration samples, self-trapped exciton luminescence at 270 nm was observed. A sharp line around 300 nm in Ce 20% doped sample was a noise due to a direct hit of X-ray or background cosmic ray to CCD. Fig. 4 demonstrates the relation between scintillation decay times and Ce concentrations. Scintillation decay times monotonically became faster when the Ce concentrations increased. Finally, scintillation decay time reached w30 ns in Ce 20% doped sample. Based on these optical and scintillation evaluations, it was confirmed that our Cedoped CaF2 crystals exhibited typical properties of this material system. To evaluate scintillation light yield, all samples were irradiated with 5.5 MeV a-rays of 241Am. Fig. 5 exemplifies relation between the scintillation light yield and the dopant concentration. As a representative, the pulse height spectrum of Ce 0.1% doped sample is demonstrated in the inset and a-ray full energy peak was clearly observed. Taking into account the quantum efficiency of the used PMT around 320 nm, scintillation light yield was deduced. Scintillation light yields monotonically decreased with Ce concentrations and this tendency was consistent with previous result (Gektin et al., 2009; Yanagida et al., 2010c).
3. Results and discussion
Intensity (a. u.)
Fig. 2 represents optical in-line transmittance spectra of Cedoped CaF2 from 190 to 320 nm. Absorption bands at 200, 240, and 300 nm were ascribed to Ce3þ 4fe5d transition and absorption depths were in proportion to nominal Ce concentrations. Transmittance at wavelength longer than 350 nm was typically 80e90% without noticeable absorption lines. Then, we investigated basic luminescence properties. Fig. 3 represents X-ray induced radioluminescence spectra of Ce differently
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Fig. 1. Ce-doped CaF2 crystals illuminated by UV-lamp.
Fig. 3. X-ray induced radioluminescence spectra of Ce-doped CaF2 scintillators in the wavelength range from 180 to 700 nm. The supplied bias voltage and tube current were 80 kV and 1 mA, respectively. The X-ray exposure time was 10 s for each integration.
Please cite this article in press as: Yanagida, T., et al., Scintillation and optical stimulated luminescence of Ce-doped CaF2, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.03.020
T. Yanagida et al. / Radiation Measurements xxx (2014) 1e4
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Ce concentration (mol%) Fig. 5. Scintillation light yield plotted against Ce concentrations. The inset shows 241 Am a-ray spectrum of Ce 0.1% doped CaF2. The shaping time was 2 ms and 5.5 MeV peak channel was determined by a single gaussian fitting.
doped CaF2 exhibited the highest OSL intensity since it had a large number of defects due to a charge imbalance of Ce3þ and Ca2þ. Then in Fig. 7, the relation between the OSL intensity after 2 Gy X-ray exposure and the dopant concentration is shown. Unlike the similar plot in scintillation light yield, OSL intensity monotonically increased with the Ce concentration. Finally, the relation between the scintillation light yield under 241 Am a-ray irradiation and OSL intensities after 0.5e4 Gy X-ray exposure is demonstrated in Fig. 8. Though it was obvious from Figs. 5 and 7, a reverse proportional relation between the scintillation light yield and the OSL intensity was clearly observed and our assumption of the complementary relation of scintillation and OSL was experimentally confirmed in Ce-doped CaF2 system. If we used X- or g-ray as an excitation source to evaluate scintillation light yield, the observed relation will be similar since charged particle and high energy photon induced light yields are in proportion but the energy conversion efficiency is different. This conversion efficiency is called a/b (or a/g) ratio and this ratio shows little dependence on dopant concentrations (e.g., the case in LuAG:Pr, Yanagida et al., 2012). Except Ce 20% doped one, OSL intensities
Stimulation wavelength (nm)
Fig. 6 depicts OSL emission map of Ce 20% doped CaF2. OSL appeared around 320 nm with 550 and 830 nm stimulation. Throughout OSL evaluations, QY was deduced by summing up these 550 and 830 nm emission peaks. Among present samples, Ce 20% 500
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Fig. 6. OSL emission map of Ce 20% doped CaF2. The horizontal axis shows the stimulation wavelength and the vertical axis exhibits emission wavelength with 10 nm step.
Fig. 8. Relation between the scintillation light yield (ph/5.5 MeV-a) and OSL intensity (QY, %) after 0.5, 1, 2, and 4 Gy X-ray exposure.
Please cite this article in press as: Yanagida, T., et al., Scintillation and optical stimulated luminescence of Ce-doped CaF2, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.03.020
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systematically decreased when X-ray exposure increased and it was suggested that OSL responses in Ce 0.1e1% doped samples saturated around few Gy exposure. Simply, OSL response was improved in higher Ce concentration sample by creation of defects due to the charge imbalance. Of course, the complementary relation of scintillation and OSL is not always observed in all material systems because there are some competing processes with OSL/TL in energy storage processes of dosimeter materials (e.g., tunnelling). In such material systems, most part of the absorbed energy would be converted to thermal form and we could not observe the complementary relation in such material systems. 4. Conclusion Ce 0.1e20% doped CaF2 crystals were prepared by Tokuyama Corp. using the simple solidification method. By optical transmittance, radioluminescence, and scintillation decay, it was confirmed that Ce-doped CaF2 used in this work exhibited typical properties of this material system. Scintillation light yield had an inverse proportionality to Ce concentrations while OSL intensity was in proportion to dopant concentrations. The hypothesis that the scintillation and OSL had a complementary relation in some material systems was confirmed experimentally. Acknowledgements This work was mainly supported by JST Sentan, A-step and partially by a Grant in Aid for Young Scientists (A) e 23686135, and Challenging Exploratory Research e 23656584 from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese government. Partial assistance from Nippon Sheet Glass Foundation for Materials Science and Engineering, Tokuyama Science Foundation, Iketani Science and Technology Foundation, Hitachi Metals Materials Science Foundation, Mazda Foundation, JFE 21st Century Foundation, and The Asahi Glass Foundation, are also gratefully acknowledged. References Gektin, A.V., Shiran, N., Nesterkina, V., Boyarintseva, Y., Baumer, V., Stryganyuk, G., Shimamura, K., Villora, E., 2009. Luminescence of heavily ce-doped alkalineearth fluorides. IEEE Trans. Nucl. Sci. 56, 1002e1005.
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Please cite this article in press as: Yanagida, T., et al., Scintillation and optical stimulated luminescence of Ce-doped CaF2, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.03.020